This invention relates to a blast furnace hearth lined with refractory bricks.
In the lower portion of a blast furnace where the molten metal collects, especially in the portion below the iron notch, the surface of the lining refractory layer is in contact with the molten metal at a temperature of 1400.degree. C. to 1600.degree. C.
The refractory layer consists of refractory bricks and is protected by external cooling. The recent increase in blast furnace volume has led to use of larger bricks and severer operating conditions involving, for example, high-pressure operation and high-rate tapping. This tendency has resulted in increased thermal wearing of the carbon bricks. Main causes for this increased thermal wearing are:
(1) The dissolution of carbonizing material from the carbon bricks into the molten iron; and
(2) The infiltration of molten iron into the carbon brick pores.
The dissolution of carbonizing material can be prevented by using a carbonizing material having low solubility or a carbonizing material having a dissolution inhibitor added thereto, as has already been carried out in various ways.
To prevent the infiltration of molten iron, it is essential to reduce the size of the carbon brick pores.
It has been estimated that molten iron can infiltrate even into small pores approximately 5 .mu.m in diameter. It has also been confirmed by the dissection of blown out blast furnaces that the infiltration occurs in pores as fine as approximately 1 .mu.m.
Obviously, the smaller the pore size, the less will be the molten iron infiltration. Reduction of the pore size to below 5 .mu.m substantially eliminates iron infiltration and prolongs the brick life greatly.
Large carbon bricks manufactured by conventional ordinary processes contain many pores approximately 10 .mu.m in diameter. These bricks are highly susceptible to iron infiltration.
A basic known method for reducing the carbon brick pore size is to produce the bricks from materials mixed at a specific grain size distribution wherein, in particular, the ratio of fine-grained particles is increased.
The increase in the ratio of fine-grained particles according to method, however, calls for increased addition of tar or other binder and this results in the formation of more pores. The increase in amount of added binder causes greater contraction on burning, which produces many cracks in the product bricks. Besides, the pore size cannot be made smaller than approximately 5 .mu.m.
For these reasons, large carbon bricks with fine pores have been difficult to obtain.